1. Field of the Invention
This invention relates to recording holograms and, more particularly, to a method of recording updateable holograms on a photorefractive polymer with a short write time at high diffraction efficiency and long persistence for 3D Holographic Displays and other applications.
2. Description of the Related Art
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was when recorded. Alternately, holograms can be computer generated by calculating the modulation pattern that would have been formed if beams with certain characteristics (wave front, intensity) would have crossed each other. The calculated pattern is next transferred to a medium to make the actual hologram. The image changes as the position and orientation of the viewing system change in exactly the same way is if the object were still present, thus making the recorded image (hologram)  appear three dimensional. The technique of holography can also be used to optically store, retrieve, and process information. While holography is commonly used to display static 3-D pictures, it is not yet possible to generate arbitrary scenes by a holographic volumetric display.
As shown in FIG. 1, when two coherent optical beams e.g. a reference beam 10 and an object beam 12 of finite beam width cross each other at a point in space, they interfere. The phase difference between the beams at each spatial location in the plane defined by the bisector of the vectors formed by the two beams defines the intensity pattern. The recording of this intensity variation into a medium 14 as a phase and/or intensity modulation results in the formation of a hologram 16. The object beam may be scattered from the object or modulated based on computer generated images to produce a 3D image. Typical media include film, photothermoplastics, photopolymers, photochoromics and photorefractive (PR) polymers.
PR polymers are dynamic holographic recording materials. As shown in FIG. 2, a recording device 20 includes a layer of PR polymer material 22 sandwiched between a ground electrode 24 on electrode support 26 and a high-voltage electrode 28 on electrode support 30. A high-voltage power supply 32 applies a voltage between ground electrode 24 and high-voltage electrode 28 to provide a bias electric field 34 across the PR polymer.
As shown in FIG. 3, in PR polymers, a three-dimensional refractive index pattern—a phase hologram 40—replicates the non-uniform interference pattern 42 formed by the two incident coherent light fields 44 and 46. This effect is based on the build-up of an internal space-charge field Es 48 due to selective transport and trapping of the photo-generated charges 50, and an electric field induced index change via the electro-optic effect. This process—in contrast to photochemical processes involved in photopolymer holograms—is fully reversible, as trapped charges can be de-trapped by uniform illumination. The erasability of the PR gratings allows for refreshing/updating of the holograms. In a typical PR material the holograms are viewed with the help of a reading beam, as long as the initial writing (recording) beams are present. When the writing beams are turned off, the PR hologram decays at a rate determined by the material properties and ambient temperature. PR polymers that have fast recording usually have high decay rates.
The quality of the PR polymer and the written hologram can be evaluated with  respect to three parameters: write time, diffraction efficiency and persistence. The write time is simply how long the writing beams must illuminate the media to record the hologram. The diffraction efficiency determines the intensity or contrast (quality) of the recorded hologram. In theory, PR polymers can achieve 100% diffraction efficiency. The persistence is a measure of the rate of decay of the hologram and is defined herein to be the time from when the write beams are removed until the diffraction efficiency falls to 10%.
As shown in FIG. 4, in PR polymer materials used in transmission holograms, the diffraction efficiency 60 according to the applied bias electric field is a periodic function. Indeed, the diffraction efficiency η is related to the index modulation Δn by the following equation: η ∝ sin2(Δn(E)), the index modulation being proportional to the applied field: Δn ∝ (Eext)P, with usually 1>P>2. While the index modulation monotonically increases with the external field, the efficiency periodically goes through maxima and minima. The diffraction efficiency as a function of the bias field is measured under steady state conditions, meaning that the field is varied slowly enough as to not perturb the efficiency by any dynamic effect. So, the efficiency is time stable at each point and the value equals the plateau observed in the dynamic measurement. The first maximum 62 of the diffraction efficiency defines the optimum field. It is the minimum field for which the efficiency is at a maximum. At lower fields the efficiency is less than at the optimum. This defines the “low field region” 64. Increasing the field above the optimum decreases the efficiency to a second minimum 66 before it increases again to a second maximum 68 and so on. This defines the “high field region” 70. Depending on the application, PR polymer materials are operated near the optimum field for best efficiency or at the highest possible field for largest two beam coupling gain. Operating a PR polymer at high field endangers the material due to the risk of dielectric breakdown and one must use caution in that domain to avoid damaging the material. Moreover, high applied fields reduce the efficiency and generate parasitic effects like second order diffraction (the reason that the second maximum is usually smaller than the first one).
FIG. 5 shows the diffraction efficiency as a function of time measured at various constant electric fields. T0 is the time when writing beams are turned on. The electric field  is turned on prior to T0 so there is no dynamic effect related to bias field build up. Td is the time when both writing beams are turned off. The dark decay is the self erasure of the hologram measured at constant field. Both writing and decay dynamics depend on the applied bias field. For a low field, the efficiency 80 ramps up slowly to reach a plateau (steady-state) equal to the diffraction efficiency for the low field in FIG. 4 when the writing beams are on. Once the writing beams are turned off, the efficiency 80 decays with a large time constant. Dark decay occurs because of thermally activated recombination of charges, leading to erasure of the internal space-charge field which ultimately reduces the index modulation and thus the diffraction efficiency. The lower the field the less steep the initial decay of the dark decay curve. For the optimal field, the efficiency 82 ramps up more quickly to reach a plateau (steady-state) equal to the diffraction efficiency for the optimal field in FIG. 4 when the writing beams are on. Once the writing beams are turned off, the efficiency 82 decays with a moderate time constant. Although the time constant for decay is smaller than for the low field because the plateau diffraction efficiency is much higher, the persistence for the optimal field is actually larger. For a high field, the initial rise of the efficiency 84 is faster than for the optimum field case but the final efficiency is less. Moreover, if the field is set above the second efficiency minimum 66 defined in FIG. 4, the time dependent diffraction efficiency reaches a first peak 86, reduces to a minimum 88 and then increases again before reaching the steady state. Although the initial time constant for the high field case is smaller than that for the optimal field, the writing time to achieve steady-state diffraction is comparable. Once the writing beams are turned off, the efficiency 84 decays with a small time constant. For all of the fields, a figure of merit (FOM) defined as the ratio of the persistence (decay time to 10% efficiency) to the write time is between 1 and 10 depending upon the polymer material. These results lead one to conclude that the optimum field value is the best setting for writing time, efficiency and persistence.
Holographic 3D displays are one application of recordable holograms. Computer generated holographic 3D displays provide highly realistic images without the need for special eyewear, making them valuable tools for applications that require “situational awareness” such as medical, industrial, and military imaging. Current commercially available holographic 3D displays employ photopolymers that lack image-updating  capability, resulting in their restricted use and high cost. Dynamic updateable 3D holographic displays based on acousto-optic, liquid crystal display and MEMS based recording media have been demonstrated. Unfortunately, these devices do not have memory, and thus do not exhibit persistence of recorded images. The lack of persistence results in the requirement of update rates faster than 30 Hz to avoid image flicker. Since 3D images exhibit very high information content, this high refresh rate requirement currently limits real time holographic displays to small sizes. Although updateable, PR polymers have not been used for 3D displays because of their large writing times and low persistence. The ultimate goal is to be able to write at high enough rates to provide near video capability. An interim goal is provide the capability to update holograms with reasonable write times, high diffraction efficiency and persistence long enough to view the display. To extend dynamic holographic 3D displays towards practical applications, alternative materials with high efficiency, reversible recording capabilities, memory, and significantly larger sizes are needed.
U.S. Pat. No. 6,859,293 to Klug et al. entitled “Active Digital Hologram Display” issued Feb. 22, 2005 discloses that certain types of holographic recording materials can be used to updateably record holographic stereograms. Klug discloses different writing configurations and does mention that pulsed lasers may shorten the write time considerably. However, this patent does not discuss techniques for achieving updateable holograms with exceptionally large persistence, and more particularly large FOM.